This invention relates to a device and method for the sustained release of a chemical onto an animal. More particularly, this invention relates to a device for the sustained release of a chemical contained within the device and to the use of the device on animals for the control of insects, parasites and pests susceptible to the chemical.
Several types of pests afflict range animals, especially herds of cattle, and can cause substantial economic loss to farmers and ranchers. Among these pests are horn flies, Haematobia irritans (L.); face flies, Musca autumnalis De Geer; the Gulf Coast tick, Amblyomma maculatum Koch; the screwworm, Cochliomyia hominivorax (Coquerel); and the southern cattle tick, Boophilus microplus (Canestrini).
In order to control these insects and pests, it is customary to treat animals by spraying the hair coats of the animals with an insecticide solution. This type of treatment provides protection against the pests for a period of about three weeks. Beyond this period of time, the insecticide treatment is usually ineffective because the insecticide was degraded by light and microorganisms. In addition, the use of relatively large amounts of the insecticide is necessitated by the need to spray the entire surface of the animal and to compensate for the gradual deterioration of the insecticide following application to the animal. These disadvantages led to the development of alternative insecticide delivery systems, such as insecticidal ear tags for animals.
Insecticidal ear tags have recently become a valuable tool for the control of livestock pests. The ear tags release an insecticide, which is spread when the animal causes the tag to rub its hair coat or the hair coat of other animals. Ear tag effectiveness has been well documented by researchers across the United States. The insecticidal ear tags are commonly referred to as controlled-release, sustained-release, or slow-release devices. The pesticide, formulated in a monolithic polymer matrix, typically polyvinylchloride, is slowly released from the matrix over a prolonged period and deposited on the hair coat of cattle. The insecticidal ear tag was rapidly accepted by the producer. In contrast to the 2-3-week intervals between spraying of cattle with liquid sprays, the rancher cold use an insecticidal ear tag to treat his cattle with a device similar to the familiar numbered identification tags and expect season-long control in most parts of the U.S.
Although originally developed for control of Gulf Coast ear ticks, the insecticidal ear tag has found greatest use in the control of the horn fly, a much more serious pest of cattle. E. H. Ahrens, Horn fly control with an insecticide-impregnated ear tag, Southwest. Entomol., 2 (1977) 8-10. E. H. Ahrens and J. Cocke, Season long horn fly control with an insecticide-impregnated ear tag, J. Econ. Entomol., 72 (1979) 215. The first commercially available tags contained an organophosphate, either stirofos or chlorpyrifos, and provided 6-10 weeks control of horn flies on cattle. Soon thereafter, tags containing pyrethroids were introduced and were shown to be capable of controlling horn flies for 16-24 weeks. R. E. Williams and E. J. Westby, Evaluation of pyrethroids impreganated in cattle ear tags for control of face flies and horn flies, J. Econ. Entomol. 73 (1980) 791-792. C.D. Schmidt and S. E. Kunz, Fenvalerate and stirofos ear tags for control of horn flies on range cattle, Southwest. Entomol. 5 (1980) 202-206. F. W. Knapp and F. Herald, Face fly and horn fly reduction on cattle with fenvalerate ear tags, J. Econ. Entomol., 74 (1981) 295-296.
Because of the wide spread use of the insecticidal ear tag, horn fly populations, particularly in the southern U.S., were subjected to greater selection pressure by a single class of insecticides than perhaps ever before. In addition, the long-lasting effectiveness of the insecticidal tag treatment meant that multiple generations were exposed to the highly selective dosages. Consequently, horn flies developed significant resistance to the pyrethroids in as little as 2-3 years after introduction of the tags. H. J. Schnitzerling, P. J. Noble, A. Macqueen, and R. J. Dunham, Resistance of buffalo fly, Haematobia irritans exiqua (de Meijere), to two synthetic pyrethroids and DDT, J. Australian Entomol. Soc. 21 (1982) 77-80. D. C. Sheppard, Fenvalerate and flucythrinate resistance in a horn fly population, J. Agric. Entomol. 1 (1984) 305-310. S. S. Quisenberry, J. A. Lockwood, R. L. Byford, H. K. Wilson and T. C. Sparks, Pyrethroid resistance in the horn fly, Haematobia irritans (L.), J. Econ. Entomol. 77 (1984) 1095-1098. C. D. Schmidt, S. E. Kunz, H. D. Petersen and J. L. Robertson, Resistance of horn flies to permethrin and fenvalerate, J. Econ. Entomol., 78 (1985) 402-406.
One method of managing resistance is to change to a class of insecticides that has a different mode of action against the pest. George P. Georghiou, Management of resistance in arthropods, in: George P. Georghiou and Tetsuo Saito (Eds.), Pest Resistance to Pesticides, Plenum Publishing Corporation, New York, 1983, pp. 769-792. Since cross-resistance has been shown between the chlorinated hydrocarbons and pyrethroids, the organophosphates (OP's) appear to be the class of choice for applying this resistance management strategy to the horn fly. However, the organophosphates are generally less toxic and tend to degrade more rapidly in the environment. For horn fly control, OP's must be applied at higher dosages and more frequently than pyrethroids. This means that a controlled-release delivery system for the OP's will have to deliver greater quantities of the insecticide than was necessary with the pyrethroids.
Most currently available ear tags are of a monolithic structure, and the release rate of insecticide decreases with time. In addition, only relatively small amounts of insecticide can be loaded into monolithic ear tags without compromising the strength of the polymer matrix. The decreasing release rate and limited loading restrict the duration of efficacy to about 2 to 4 months. As efficacy declines, the ear tag continues to release insecticide, but at sub-lethal doses. It has been reported that more than 50% of the insecticide loaded into monolithic ear tags is still in the ear tags after the ear tags lose their effectiveness. This is not only an inefficient use of the insecticides, but increases the likelihood that fly populations will develop resistance as a result of prolonged exposure to sub-lethal doses of the insecticides.
Thus, there exists a need in the art for a delivery system for insecticide compositions, such as OP's, in which the release rate is sustainable and controllable. The delivery system should be capable of providing relatively high loadings of the insecticide composition. Ideally, the delivery system should be capable of delivering the composition at a level necessary for control of pests for durations equal to the pyrethroid ear tag. In addition, the delivery system should have a predictable and reproducible release rate profile so that the release rate will not decline to such a level that sub-lethal doses of the insecticide composition are released for substantial periods of time.